CA1236405A

CA1236405A - Spectral emphasis and de-emphasis

Info

Publication number: CA1236405A
Application number: CA000464817A
Authority: CA
Inventors: Kenneth J. Gundry; Craig C. Todd
Original assignee: Dolby Laboratories Licensing Corp
Current assignee: Dolby Laboratories Licensing Corp
Priority date: 1983-10-07
Filing date: 1984-10-05
Publication date: 1988-05-10
Also published as: EP0138547A3; DK478984D0; NO844015L; ES8704681A1; NO172669B; DE3485473D1; ES8607646A1; KR850003086A; KR950015080B1; NO172669C; DK169693B1; ES548972A0; ES548973A0; BR8405048A; DK478984A; ES8704682A1; EP0138547B1; ES536553A0; EP0138547A2

Abstract

ABSTRACT
This invention is advantageous for reducing noise introduced by a transmission medium whose noise level rises with the signal level in the medium. The spectral emphasis circuit of the invention analyzes the spectral composition of an input signal and generates an emphasis control signal which indicates the spectral composition of the input signal and the regions in the frequency spectrum, if any, where the predominant components of the input signal are concentrated. The emphasis control signal is substantially independent of the amplitude of the input signal. The emphasis control signal is also limited by a limitation circuit to a desired bandwidth. The spectral emphasis circuit further comprises means for altering the spectral composition of the input signal by applying to the spectral components of the input signal, emphasis of varying amounts as a function of the emphasis control signal. A delay circuit delays the input signal relative to the emphasis control signal so that the input signal reaches the spectrum altering circuit when the emphasis control signal is available for controlling the emphasis applied. The spectral de-emphasis circuit restores the spectral composition of the input signal. The spectral de-emphasis circuit comprises means for deriving a de-emphasis control signal of limited bandwidth from the emphasis control signal. The de-emphasis circuit further includes means for applying de-emphasis to the spectral components of the altered signal as a function of the de-emphasis control signal. The de-emphasis applied by the restoring means is comple-mentary to the emphasis applied by the spectral emphasis circuit. Since the de-emphasis control signal has a limited bandwidth, the de-emphasis applied can only change slowly. The effect of transmission errors is reduced. The emphasis control signal also has a limited bandwidth. Hence both the emphasis and de-emphasis change slowly, and the system is tolerant of component and timing errors as well.

Description

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This application is related to another Canadian applica-tion file herewith and referred to below as the companion application "A-D Encoder and D-A Decoder System" by Craig C. Todd and Kenneth James Gundry, Serial No. 464,81a filed October 5, 1984.
Background of the Invention . .
This invention relates in general to emphasis and de-emphasis circuits and in particular to circuits which reduce noise by altering the spectral content of the signal.

In many adaptive A-D and D-A coding systems the step-size used increases with the level of the input signal. Since quantizing noise increases with the step-size the quantizing noise of such adaptive sys-tems increases with the input signal level, an effect known as noise modula-tion. The eEfects o:~noise modulat.ion is distu.rbing in many applications, such as in high quality audio.

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It is a characteristic of human hearing that spurious spectral information is much less audible if it is close in frequency to the desired audio signal.
If the spurious enerqy lies far from the desired audio signal frequency it is ~uch more likely to be audible.
Thus where the noise level is a function of the input signal level it is particularly important to reduce noise whose frequencies are far from the desired audio signal.
Conventional noise eeduction systelns have been used to apply adaptive emphasis and de-emphasis to reduce audible noise. One conventional system employs fixed emphasis Eor boosting high frequency signals and complementary de-emphasis for bucking such signals.
When such emphasis and de-emphasis are used to reduce audible noise that increases with signal level and when the predominant signals are of high frequencies, low frequency noise will instead be increased. Fixed high frequency emphasis and cle-emphasis are therefore unsatisfactory for reducing such noise.
A well known type of circuit, called "sliding band", reduces audible high frequency noise by way of a filter with a variable corner frequency. As the level of high frequency signals increases, the filter corner frequency slides upwardly to narrow the band boosted and cut. Examples of such circuits are to be fo~nd in VS-PS Re 2B,426, US-PS 4,072,914 and US-PS 3,934,190.
The sliding of the filter corner frequency depends on both the amplitude and frequency of the input signal. If such a "sliding band" type circuit is used to reduce audible noise that is a function of input signal level, low frequency noise may also be increased when the predominant spectral components of ~:36~5 the input signal are at v~ry high frequencies. Whil2 such a problem is not as serious as in the case of the high frequency fixed emphasis and de-emphasis, the "sliding band" type circuit is not entirely satisfactory for reducing noise which increases with signal level.

Summa y_~f The Invention The invention is based on the observation that in order to reduce noise whose level is a Punction oF the input signal level, the diffeeent spectral components oP the input signal may be altered depending on the spectral composition of the input signal by subjecting the input signal to emphasis and su~sequent de-emphasis in a complementary manner to recover an output signal which is substantially similar to the input signal. The spectral emphasis and de-emphasis circuits of this invention are particularly advantageous for reducing audible quantizing noise in many adaptive A-D and D-A coding systems.
~he apparatus of this invention comprises a circuit for altering the spectral composition of an input signal and a circuit for restoring the spectral composition of the signal. The circuit Por altering the spectral composition of the input signal comprises means for analyzing the relative spectral composition oP the input signal and for identifying the regions in the frequency spectrum, if any, where the predominant components of the input signal are concentrated. The analyzing means generates an emphasis control signal oP
limited bandwidth to indicate the spectral composition and such regions. The circuit further comprises means responsive to the emphasis control signal for altering ~36~

the spectral composition of the input signal to generate an output signal by applying to the spectral components, emphasis of varying amounts as a function of the emphasis control signal.
The circuit for restoring the spectral composition of the input signal receives from the spectral composition altering circuit an altered signal and spectral information of the signal through a transmission medium to restore the spectral composition O~ the signal. The restoring means comprises means for generating Erom the spectral inEormation a de-emphasis control signal for restoring the spectral composition o~ the signal by applying de-emphasis to the spectral components of the altered signal as a function of the de-emphasis control signal. The de-emphasis applied by the restoring means is substantially complementary to the emphasis applied by the spectral composition altering circuit.
In one embodiment of the restoring circuit, the de-emphasis control signal is limited in bandwidth be~ore it is used to control the de-emphasis applied to reduce noise caused by transmission errors. In a second embodiment, the spectral information is received by the restoring circuit in advance of the altered signal by a predetermined and substantially fixed time interval. Such time interval permits bandwidth limitation o~ the de-emphasis control signal in the restoring circuit by compensating for the time delay caused by such bandwidth limitation, so that the bandwidth limited de-emphasis control signal reaches the de-emphasis applying means at substantially the same time as the altered signal.
Another aspect of the invention is direct:ed to a particular emphasis and complementary de-e~phasis ~a~3~

which, when applied to a medium whose noise level is a function of the signal level in the medium, reduces noise intro-duced by the medium. In such aspect, an emphasis control siynal is generated to indicate the spectrum of the input signal and the regions in the frequency spectrum, if any, where pre-dominant components of the input signal are concentrated. The altering circuit compri~es means responsive to the emphasis control signal for applying emphasis to the spectral components by boosting the components with frequencies above a variable frequency while buc~ing or leaving substantially unchanged the components with frequencies below such variable frequency.
Such variable frequency is a characteristic of the emphasis applying means; it slides substantiall.y continuously upwards in requency when the predominant signal components o:E the input sigllal rise in frequency so that it is above the frequencies of the predominant signal components.
The restoring circuit has characteristics which are substantially complementary to those of the altering circuit.
Such characteristics of the altering and restoring circuits are particularly advantageous for reducing noise which increases with signal level as well as signal frequency.
Thus, in accordance with a broad aspect of the invention, there is provided a circuit for pre-processing a sig-nal before the signal is subjected to a medium which introduces noise, said circuit altering the spectral composition of the signal before it is subjected to th.e medium to reduce the noise subsequently introduced, said circuit comprising:

means for analyzing the spectral composition of said signal, and for generating a control signal indicative of the regions in a frequency spectrum, if any, where the predominant . . ~

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--5a- 27332-30 signal components of -the signal are concentrated, wherein said analyzing means includes a bandwidth limitation circuit for limiting the bandwidth of the control signal;
means responsive to said control signal for altering the spectral composition of said signal by amplifying or attenuating the spectral components of said signal by different amounts as a function of the control signal; and means for introducing delay to said signal before reaching said spectral composition altering means to compensate for the bandwidth limitation, wherein the noijse subsequently introduced by the medium is reduced.
In accordance with another broad aspec-t o:E the invention there is provided a circuit Eo.r res-toring the spectral composition oE a siynal received f.rom a medium whose noise level varies with the signal level, said circuit receiving through the medium the signal whose spectral composition has been altered by the application of emphasis to its spectral components by different amounts as a function of spectral information indicative of regions in a frequency spectrum, if any, where the predominant signal components of the signal are concentrated, said circuit also receiving through the medium the spectral information, wherein the spectral information received is distinguishable from the signal, said circuit comprising: .
means responsive to said spectral information for genera-ting a de-emphasis control signal, said generating means including a bandwidth limitation circuit for limiting the bandwidth of the de-emphasis control signal; and means responsive to the bandwidth limited de-emphasis control signal for restoring said spectral composition bv j~ .

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-5b- 27332-30 applying de-emphasis to the spectral components of the altered signal, and wherein the effects of errors introduced by the medium are reduced.
In accordance with another broad aspect of the invention there is provided a system for pre-processing a signal to prepare i-t for a medium which introduces noise and for post-processing the signal after it has been subjected to the medium, said system comprising:
(a) a pre-processing circuit which alters the spectral composition of a signal comprising:
means for analyzing the spectral composition of said signal, and for generating an emphasis control signal indicative of the regions in a frequency spectrum,if any, where the predominant components of the signal are concentrated, wherein said analyzing means includes a circuit for limiting the bandwidth of the emphasis con-trol signal;
means responsive to said emphasis control signal for alter-ing the spectral composition of said signal by applying emphasis to the spectral components of said signal by different amounts as a function of the emphasis control signal to prepare it for the medium; and means for introducing delay to said signal before reaching said spectral composition altering means to compensate for the : time required for limiting the bandwidth of the emphasis con-trol signal, and (b) a post-processing circuit for restoring the spect-ral composition of the signal, said circuit receiving the signal after it has been subjected to the medium, said post-processing circuit also receiving through the medium 3~ the emphasis control signal or a signal derived from said , Z~3~3L~J5 -5c- 27332-30 emphasis control signal, said post~processing circuit comprising:
means responsive to said emphasis control signal or the signal derived therefrom for generating a de-emphasis control signal, said de-emphasis control signal generating means including a circuit for limiting the bandwidth of the de-emphasis control signal; and means responsive to the de-emphasis control signal for restoring said spectral composition by applying de-emphasis to the spectral components of the altered signal, so that the noise introduced by the medium is reduced.
In accordance wi-th another broad aspect of the invention there i.s prov.ided a circuit for pre-processing a signal to prepare it for a meclium whose noise level :rises with the signal level, said circuit comprising:
means for analyzing the spectrum of the signal and for generating a control signal indicative of the regions in a frequency spectrum,if any, where the predominant components of the si:gnal are concentrated; and means responsive to said control signal for applying emphasis to the spectral components of the signal, so that when the predominant signal components are concen-trated within a first frequency range, at least some of the spectral components with frequencies above those of the pre-dominant spectral components are boosted, and the predominant spectral components remain substantiall~ unchanged and when the predominant si~nal components are concentrated within a second frequency range with frequencies above those of the first range, spectral components with frequencies above those of the predominant signal components are boosted but the predominant signal components are bucked, so th.at when the pre-dominant signal components are concentrated in -the second -5d- 27332-30 frequency range, the rise in noise level caused by the medium is reduced.
In accordance with another broad aspect of the invention there is provided a circuit for restoring the spectral composition of signals by applying variable de-emphasis, said circuit receiving through a medium a signal whose spectral composition has been altered by the application of emphasis to its spectral components by different amounts as a function of the spectral composition of the signal and in response to an emphasis control signal before it is subjected to the medium, said emphasis control signal indicating regions in a frequency spectrum, if any, where -the predominant components of the signal are concentrated, said medium having a noise level which rises with the signal level, said circuit also receiving through the medium spectral information of the s:ignal, wherein said spectral inEormation received is distinguishable from the corresponding signal, said circuit comprising:
means responsive to said spectral information for generating a de-emphasis control signal; and means responsive to said de-emphasis control signal for applying de-emphasis to the spectral components so that, when the predominant signal eomponents are coneentrated within a first frequeney range, at least some of the speetral eomponents with frequeneies above those of the predominant speetral eomponents are bueked, and the predominant spectral components remain substantially unchanged and when the predominant signal components are concentrated within a second frequency range with frequeneies above those of the first range, spectral components with frequencies above those of the predominant signal components are bucked but the predominant signal components are ~oosted, 36'~
-5e- 27332-30 so that when the predominant signal components are concentrated in the second frequency range, the rise in noise level caused by the medium is reduced.
In accordance with another broad aspect of the invention there is provided a system for pre-processing a signal to prepare it for a medium whose noise level rises with thesignal level, and for post-processing the signal after it has been sub-jected to the medium, said system comprising:
(a) a pre-processing circuit which alters the spectral composition of the signal to prepare it for the medium, said pre-processing circuit comprising:
means for analyzing the spectrum of the signal and for generating an emphasis con-trol signal indicative of regions in a frequency spectrum where the predominant components o:E the signal are concentrated; and means responsive to said emphasis control signal for apply-ing emphasis to the spectral components of the signal, so that when the predominant signal components are concentrated in a first frequency range, at least some of the spectral compon-ents with frequencies above those of the predominant spectral components are boosted, the predominant spectral components remain substantially unchanged and when the predominant signal components are concentrated in a second frequency range with ~
frequencies above those of the first range, spectral components with frequencies above those of the predominant spectral com-ponents are boosted but the predominant signal components are bucked; and (b) a circuit for post-processing the signal after it has been subjected to the medium to restore its spectral composition by applying variable de-emphasis, said circuit receiving through , -5f- 27332-30 the medium the emphasis control signal or a signal derived therefrom, said post-processing circuit comprising:
means responsive to said emphasis control signal or the signal derived therefrom for generating a de-emphasis control signal; and means responsive to said de-emphasis control signal ~br applying de-emphasis to the spectral components so that when the predominant signal components are concentrated in the first range, at least some of the spectral components with frequencies above those of the predominant spectral components are bucked, and the predominant spectral components remain substan-tially unchanged and when the predomi~ant signal components are concentrated .in the second range, spectra:L
components with Ere~uenies above those of the predominant spectral components are bucked but the predominant signal components are boosted, so that noise introduced by the medium is reduced.
In accordance with another broad aspect of the invention there is provided a circuit for restoring the spectral composition of a signal~ said spectral composition having been altered as a function of an emphasis control signal, said emphasis control signal indicating regions in a frequency spectrum where the predominan-t components of the signal, if any, are concen-trated, said circuit receiving the altered signal and spectral information of the signal through a medium wherein said spectraL
information is received by the circuit in advance of the altered signal by a predetermined and substantially fixed time interval, wherein said information is distinguishable from said altered signal, said circuit comprising:

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-5g- 27332-30 means responsive to said spectral information for genera-ting a de-emphasis control signal, said generating means includ-ing means for limiting the bandwidth of the de emphasis control signal; and means responsive to the bandwidth limited de-emphasis control signal for restoring said spectral composition by applying de-emphasis to the spectral components of the altered signal;
wherein said substantially fixed time interval compensates for the rise time of the bandwidth limitation so that the bandwidth limited de-emphasis control signal is available to the de-emphasis applying means when the spectrally altered signal related to such de-emphasis control signal arrives at the de-emphasis applying means, and wherein the effects of errors introduced by -the medium are reduced.
In accordance with another broad aspect of the invention there is provided a circuit for al-tering the spectral composition of a signal for use in a system, said system illcluding a circuit for restoring the spectral composi-tion of spectrally altered signals received through a medium, said restoring circuit also receiving -through the medium spect-ral information related to the spectrally altered signals received, said restoring circuit including means for bandwidth limiting the spectral information and means for applying de-emphasis to the received spectrally altered signals in response to the bandwidth limited spectral information related to such signals to restore the spectral compositions of such signals, said altering circuit comprising:
means for analyzing the spectral composition of a si.gnal, and for generating a control signal indicative o~ the spectral composition of said signal;

~3~ 5 -5h- 27332-30 means responsive to said control signal for altering the spectral composition of said signal by applying emphasis to the spectral components of said signal by different amounts as a function of the control signal to provide a spectrally altered signa~; and means for introducing delay to said signal before reaching said spectral composition altering means, said time delay being such that the con-trol signal is available to the medium by a predetermined and substantially fixed time inter-val before the spectrally altered signal is available to the medium;
wherein said substantially fixed time interval compen-sates for the rise time of the bandwidth limiting means so that the bandwidth limitecl spectral informa-tion is available to the de-emphas:is applying means when the spectrally altered signal related to such information arrives at -the de-emphasis means in the restoring circuit, and wherein the effects of erros introduced by the medium are reduced.
Brief Description of the Drawings .
Figure 1 is a graphical illustration of the noise and distortion emerging from an ADM coder-decode~ as a ~unction of the.step-size applied by the coder-decoder.
Figure 2A is a block diagram of an encoder which includes a pre-processing circuit illustrating an ~,.. ~

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embodiment of this invention.
Fig. ~B. is a block diagram of a decoder which includes a post-processing circuit illustrating the preferred embodiment of this invention.
Figs. 3A and 3B are graphical illustrations of respectively the pre-emphasis characteristics of the pre-processing circuit of Fig. 2A and the de-emphasis characteristics of the post-processing circuit oE Fiy.
2B.
Fi~.4 is a block diagram illustrating the preferred embodiment for a portion of the pre-processing circuit of Fig.2A.
Fig. 5 is a block diagram of a decoder which includes a post-processing circuit, the diagram containing the system definition of the decoder to illustrate the preferred embodiment of the invention.
Fig. 6 is a schematic circuit diagram for the circuit of Fig. 5 to illustrate the invention.

Detailed Descri~tion Of The Preferred Embodlment The spectral composition altering and restoration circuits of this invention are particularly suitable Eor reducing noise which is a function of signal level. Below is a description of an A-D and D-A
conversion system whose noise level rises with the signal level: the description will serve as a background desirable for understanding this invention.
Figs. 2A and 2B are block diagrams for an adaptive A-D
encoder and D-A decoder system which includes a pre-processiny circuit and a post-processing circuit for altering and restoring the spectral composition of signals to illustrate this invention. The derivation, transmission and processing of step-size inEormation in .

the encoder-decoder system is the subject of the companion application. The discussion that immediately follows concerning step-size determination is taken from such companion application.
The noise and distortion einerging from an ADM
encoder-decoder system ~codec) depend on the audio input signal and the step-size, both of which are varying. Consider a codec nandling a single sine wave.
As a function of step-size, the output noise and distortion will vary as shown qualitatively in Fig. l.
In the region labelled A, the step-size is too large, which produces excessive quantizing noise. In region 8 the step-size is too small and ~he system is in overload WhiC~ produces high noise and distortion.
There is an optimum value of step-size Eor the particular input condition labelled C. For each short time segment of real audio there is a curve like that of Fig. l, and an optimum step-size. In a conventional output controlled ADM system the step-size rarely achieves the optimum value, but remains in region most of the time, m`oving~into~region~~ on signal transients. The object oE this invention is to design an AnM system that operates as much as possible in region C, and that operates so that the delta modulator is fully loaded. This is possible because the step-size determination is done in the encoder and is input controlled as explained below.
Fig. 2~ is a block diagram for an encoder illustrating an embodiment of this invention. As shown in Fig. 2A, an analog audio input signal 12 is passed through a low-pass filter 14 to determine the overall audio bandwidth of the analog input signalO Typically such bandwidth may be 15 kHz. The analog input signal ~- f 3~

is then passed through a pre-processing circuit 16.
~he function of the pre-processing circuit lo will be discussed belo~!
After being pre-processed the analog auclio input signal is supplied to a step-siæe derivation circuit l8 and to a delay circuit 20. In one particular application the step-size derivation circuit 18 comprises a slope detector ~or detecting the time derivative or slope oF the incominq audio input signal.
The slooe d~ ector then generates a control signal indicative of the step-size to be used in the adaptive delta modulator 22. The control signal is limited by a bandwidth limitation circuit 24 and then applied to the adaptive delta modulator 22. A-D converter 26 converts the step-size control signal into a bit-stream of digital signals to convey step-size information. After being time delayed by delay circuit 20 the audio input signal is converted into a bit-stream of digital audio signals by adaptive delta modulator 22 in accordance with the step-size indicated by the bandwidth limited step-size control signal 45. The audio bit~stream and the step-size information bit-stream are then transmitted through a medium to a decoder which is shown in Fig. 2B. In one particular application encoder 10 is part of a broadcasting station transmitting the audio and step-size information bit-streams to decoders in consumer systems. ~he function of delay circuit 20 and bandwidth limitation circuit 24 will be discussed after a brief description of the decoder of Fig. 2B below.
Fig. 2B is a biock diagram of a decoder illustrating the preferred embodiment of this invention. As shown in Fig. 2B decoder ~0 comprises an ~36~5 adaptive delta demodulato~ 42 for receiving the digital audio bit-stream 30 transmitted through the medium and a D-A converter 44 for receiving the digital step-size information bit-stream 2~. D-A converter 44 converts the digital bit-stream into an analog step-size control signal ~hich is supplied to bandwidth limitation circuit 46. After being limited in bandwidth, the step~size control signal is applied to adaptive delta demodulator 42. ~daptive delta demodulator 4~
c~ener~tes an analog audio out~ut signal erOm t~e audio bit-stream in accordance with band~idth limited step-size control signal from bandwidth limitation 46.
Bandwidth limitation circuit 24 of Fig. 2A limits the bandwidth of the step-size control signal applied to adaptive delta-modulator 22 so that the step-size cannot change abruptly from one sample to the next.
Similarl~, bandwidth limitation circuit 46 limits the bandwidth of the step-size control signal applied to adaptive delta-demodulator 42. Thus if the transmission medium introduces a bit error in the step-size information bit-stream, such an error, after being converted into analog form by D-A converter 44, cannot introduce a major error in the step-size at adaptive delta demodulator 42. The effect of transmission errors is thereby reduced. Therefore, cheap non-precision components may be used to construct the converters 26, 44 and the transmission of the step-size information bit-stream is highly tolerant of bit errors.
The generation of a limited bandwidth step-size control signal will require a fini`te time. To compensate for such time, delay circuit 20 introduces a time delay so that the pre-processed analog audio input , .

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~36~5 signal will reach adaptive delta modulator 22 at a time when the step-size control signal for such input signai is available from bandt~idth limitation circuit 24.
~his is particularly advantageous when there are sudden changes in the slope of the input audio signal.
~ hile in Figs. 2A and 2B the step-size infor-mation bit-stream 23 and audio bit-stream 30 are shown as being transmitted separately, it will be understood that the two bit-streams may be transmitted together in one single channel provided that the two bit-streams can be distinguished erOm each other. 5imilarly, all three bit-streams 28, 30 and ~2 may also be transmitted in the same channel instead oE in separate channels ~f they can be distinguished Erom each other.
3y introducing a time delay through delay circuit 20 that also compensates for the time delay caused b~ the bandwidth limitation circuit 46 in the decoder, the audio bit-stream signals which are the digital representation of a particular analog signal reach the delta demodulator when the bandwidth limited step-size control signal is available from circuit 46.
In such manner, the need for a delay circuit in the decoder to compensate for the time delay caused by the bandwidth limiting of the step-size control signal in the decoder is eliminated and the decoder circuit is simplified. This is particularly advantageous for lowering the cost of consumer decoder equipment.
The invention of this application will now be discussed. The pre-processing and post-processing circuits 16, 96 and other associated circuit components illustrate this invention. Since the step-size in the A-D and D-A conversions is variable, the noise amplitude will be modulated depending on the step-size f ~3~5 and such noise modulation is undesirable in many applications such as in high quality audio equipment.
Noise modulation is reduced 'oy pre-processing circuit 16 and post-processing circuit 96 when combined with 5 components A-D converter 72 and digital delay 74 of Fig. 2A. The pre-processing circuit 16 comprises spectral analysis circuit 52, bandwidth limitation circuit 54, adaptive pre-emphasis circuit 56, and delay circuit 58, all of Fig. 2A. The post-processing 10 circuit 96 comprises D-A converter 76, adaptive de-emphasis circuit 7~ and bandwidth limitation circuit 30, all oE ~ig. 2B.
Spectral analysis circuit 52 analyzes the audio input signal to generate an emphasis control 15 signal. The emphasis control signal generated is a function only of the spectrum of the input audio and is substantially independent of the amplitude of the input audio. The emphasis control signal is then limited by bandwidth limitation circuit 54 and applied to adaptive 20 pre-emphasis circuit 56.- Adaptive pre-emphasis circuit 56 boosts or boosts and bucks the different frequency components of the input audio signal by amounts which are functions of the emphasis control signal. The emphasis control signal is limited in bandwidth so that 25 the frequency response of the adaptive pre-emphasis circuit 56 will not change suddenly from sample to sample. Bandwidth limitation circuit 80 reduces the effect of bit errors introduced by the transmission medium in a manner similar to bandwidth limitation 30 circuit 46 described above.
In reference to Figs. 2A, 2B, delay circuit 58 introduces a time delay which permits the pre-emphasis ~3~

circuit 56 to comDlete its adaptation before the audio input signal is supplied to the pre-emphasis circuit 56. The pre-processed audio input signal is supplied to delay ciecuit 20 and adaptive delta modulator 22 as described above. The emphasis control signal from spectral analysis circuit 52 is converted into a digital bit-stream carrying spectral information by A-D
converter 72 and is delayed by digital delay 74 by a time period substantially equal to that of delay circuit 20.
Comparing the timing relationship of the audio and the spectral information bit-streams, an audio signal in the audio bit-stream has been delayed by delay circuits 58, 20, whereas the corresponding lS spectral information signal for such audio signal has been delayed by only the digital delay 74. Thus the net effect is that the audio is delayed relative to the corresponding spectral information by the time delay caused by delay 58, so -that the spectral information reaches the decoder of Fig. 2B and the post-processing circuit 96 in advance of the corresponding audio signals and at the proper time to change the amplitudes of the various requency components of the audio signal in a manner complementary to that of the adaptive pre~
emphasis circuit 56. The further requirements for complementarity are discussed below. The time delay introduced in the encoder by circuit 58 compensates for the time delay caused by bandwidth limiting the spectral information in bandwidth limitation circuit 80 ~- 30 in the decoder.
If the audio and the correspondinq spectral information were synchronous so that they reached the decoder and the post-processing circuit at the same .

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-]3-time, the time delay caused by bandwidth limitation circuit ~0 would cause the audio to reach the adaptive de-emphasis 78 before the bandwidth limited de-emphasis control signal was available for controlling the de-emphasis. A delay circuit would then be required inthe decoder to delay the audio so that the audio would reach the de-emphasis 78 at the appropriate time. By introducing a timing discrepancy between the audio and the corresponding spectral information in the encoder in the manner discussed above, the need for a delay circuit in the decoder equipment is eliminatecl and the cost of the decoder is reduced.
One of the purposes oE the encoder-decoder system of Figs. 2A and 2B is to transmit an analog audio signal through a medium so that the analog audio output signal recovered after the transmission is substantially the same as the input audio. To accomplish this purpose, the adaptive delta modulator 22 in the encoder of Fig. 2A and the adaptive delta demodulator 42 are substantially complementary to each other. In addition, the step-size control signals applied to the modulator 22 and demodulator 42 are substantially the same and are applied to the modulator and demodulator with substantially the same timing relationship with respect to the corresponding audio signals so ~hat the modulation and demodulation applied are complementary. In other words, if the step-size control signal for an audio signal is applied at a time t before or after the audio signal reaches the modulator 22, the step-size control signal should reach the demodulator 42 also at substantially time t before or after the audio signal reaches the demodulator.
This assures that the modulation and demodulation ~36'~5 applied are substantially complementary. Similarly, the pre-processing and post-processing circuits are also substantially complementary to each other; the emphasis and de-emphasis control signals are substantially the same and have substantially the same timinq relationship with respect to the audio for the application of pre-emphasis and de-emphasis so that the pre-emphasis and de-emphasis applied are substantially complementary.
A~ter the above requirements ~or conplementarity are generally met, however, the encoder-decoder system is highly tolerant oE timing errors. Since the step-size, pre-emphasis and de-emphasis control signals can only change slowly in the encoder and decoder, the adaptive modulation and demodulation together with the pre-emphasis and de-emphasis applied by the encoder and decoder can only change slowly. Thus mismatch in timing relationship of the type discussed above and of the order of a few percent o-f the bandwidth limitation rise times will not cause the modulation and demodulation to deviate significantly from being complementary. Similarly mismatch in timing relationship of such order of magnitude will not cause the pre-emphasis and de-emphasis applied to deviate significantly from beingcomplementary.
The characteristics of adaptive pre-emphasis and de-emphasis circuits 56, 78 are illustrated in Figs. 3A, 3B. It will be understood that the specific Erequencies and gains in Figs. 3A, 3B and in the discussion below are for illustrative purposes only and that the characteriStiCS of circuits 56, 78 are not limited thereby. In some respects, the pre-emphasis s and de-emphasis characteristics are similar to the well known type of "sliding band" circuits, which reduce high frequency noise by way of a filter with a v~riable corner frequency. ~s the signal level increases, the filter corner frequencies o~ such "sliding band"
circui~s slide continuously and upwardly to narrow the band boosted and cut. Examples of such circuits are found in US-PS Re 28,426; ~S-PS ~,072,914 and US-PS
3,93~,190.
The pre-empnasis characteristic o~ circuit 56 also has a variable erequency indicated at S6a, 8~a, 9Qa, 92a, 9~a and 96a of the pre-emphasis characteristic curves 86, 88, 90, 92, 94 and 96 respectively of Fig. 3A. The de-emphasis curves 84'-96' of Fig. 3B are complementary to curves 84-96 respectively and also have variable frequencies 86a'-96a'. Such variable frequencies also continuously shiEt as a function of the input audio. However, unlike the "sliding band" circuits, the continuous shifting is determined, not by the level oP high frequency signals but by the spectral content of the input audio in a manner described below. In the above referenced "sliding band" circuits, the signal components with frequencies higher than the variable corner frequency are boosted (or cut) and those with frequencies lower than the corner frequency remain unchanged. While signals with frequencies higher than the variable frequency are also boosted by circuit S6 as shown in Fig. 3A, for each of curves 90 through 96, there is a spectral region in which signals are bucked.
Similarly there is a spectral region for each of de-emphasis curves 90'-96' in which signals are boosted.
The detailed characteristics of circuit 56 are described below.

~36'~C~5 It is assumed first, for the purpose of discussion, that the predominant signal components of the input audio are concentrated in a certain region of the frequency spectrum. ~hen the audio input signal comprises mostly low and middle frequency energy e.g.
concentrated in the frequency region below 500 Hz, adaptive pre-emphasis circuit 56 adopts the response labelled 8~, boostinq only signals with ~requencies above 500 Hz; the predominant signals with ~requencies below ~00 I-lz remain substantially unchanged. When the audio si~nal ~rom the adaptive delta-demodulator 42 reaches adaptive de-emphasis circuit 78 the high frequency components of the quantizing noise will be reduced by adaptive de-emphasis circuit 78 which will have a characteristic 84' complementary to curve 84 as shown in Figs. 3A and 3B~ High frequency noise above 500 Hz is thereby reduced sufficiently that audible noise modulation becomes much diminished. Low and medium frequency noise below 500 Hz i5 masked by the signal.
As the frequency of the input audio signal rises so that the predominant signal components are concentrated between about 500 Hz and 2 kHz, the emphasis control signal from the spectral analysis circuit 52 causes the frequency response of adaptive pre-emphasis circuit 56 to slide from 84 to 86 or 88.
Such dynamic action of the adaptive pre-emphasis circuit prevents undesirable increases in the step-size of the adaptive delta-modulator but still allows the subsequent complementary de-emphasis to reduce noise at fre~uencies above those of tne input signal. Low frequency noise is not yet an audible problem.
The frequency responses o~ the shape 84, 86, 88 ~that is, sliding high frequency boost) are ~, ~ ~ ~ t satisfactory For noise reduction when the predominant spectral components of the input audio signal are belo-~2 or 3 kHz. Noise at frequencies above these predominant spectral components is reduced as described above; lower frequency noise is masked by the signal.
;~1hen tne predo,~inant spectral components of the input audio signal are at high frequencies (e.g.above 3 k~z~
such sliding boost responses may no longer be satisfactory for noise reduction, since low and medium ~requenc~ noise is no longer maslced by the si~nal.
Under these signal conditions the ef~ect of high frequency boost would be to increase the step~size employed in the adaptive delta modulator 22 and demodulator 42, resulting in an increase in wide-band quantizing noise. The subsequent complementary high frequency cut would not reduce the low frequency part of this increased noise. ThuS low frequency noise would be modulated by changes in the high frequency components of the input audio signal. ~nder such .
conditions, it is desirable to convert the high frequency boost of adaptive pre-emphasis circuit 56 for the spectral region where the predominant signal components of the input audio are concentrated into a cut such as the dips shown as portions 90b, 92b, 94b, 96b of respective curves 90, 92, 94 and 96 in Fig.3A.
Therefore, as the frequencies of the predominant spectral components o~ the input audio signal rise, the frequency response of adaptive emphasis circuit 56 will slide past the curves 84, 86 and 88 to curves 90, 92, 94 and 96.
When the predominant signal components are concentrate~ in high frequencies such as around 5 kHz, ; high frequency noise around 5 kHz is masked. Noise at still higher ~requencies may not be masked and it may . .

~6'~

be desirable to reduce such noise while also reducinq low frequency noise in the manner described above.
Thus the curves 90, 92, 94 and 96 at frequencies above the variable frequency retain the shape of a high frequency shelf. ~s shown in Fig. 3A, curves 84, ~6, 88 tend towards tne same fixed qain teg. 20 dB) at high frequencies. Even though not clearly shown in Fig. 3~, curves 90-96 also tend towards the same Eixed gairl at still higher erequencies. 'rhe complementary de-emphasls curves 8~'-96' corresponding to respective pre--emphasis curves ~4-96 are shown in Fig. 3~, and have variable frequencies 86a'-96a' which are substantially the same as those of the pre-empnasis curves. De-emphasis curves 90'-96' have peaks 90b'-96b' corresponding to dips 90b-96b of the pre-emphasis curves of Fig. 3A.
The overall effect of the curves 90-96 can now be described. Pre-emphasis curves with dips at the spectral regions of the 2redominant signal components will reduce the step-size and hence the broad-band noise emerging from the encoder-decoder system. The subsequent de-emphasis peaks 90b', 92b', 94b' and 96b' will pick out the wanted predominant signal components and restore them to their original amplitudes. The de-emphasis will also buck the siynals at frequencies above the variable frequencies to reduce very high frequency noise. Thus the reduced low frequency noise level is retained, high frequency noise is masked and very high frequency noise is reduced.
In the above discussion, it has been assumed that the predominant signal components of the input audio are concentrated in a certain region of the frequency spectrum. Such an input signal is in fact the most critical case. When the signal spectral components are more distributed, their masking ~L~3~A~

properties cover more of the noise, and the shapes of the pre-emphasis curves are less critical. If the signal spectral components are distributed in two regions of the frequency spectrum, the pre-emphasis curve will resemble the curve for the case where the spectral components are concentrated in a region between such two regions.
~ andwidth limitation circuits 24,46, 59 and 8n limit the step-size and spectral control signals to within bandwidths oE a few tens or low hundreds o~ llz;
hence the control signals can have rise times oE a few milliseconds. The delay introduced by delay circuits 20, 58 is therefore chosen to be substantially equal to the rise times oP the control signals as determined by the bandwidth limiting. Suitable values are in the range 5 to 20 milliseconds. The control A-D and D-A
converters 26, 44, 72 and 76 may be simple delta or delta-sigma modulators and demodulators operating at a few kilobits per second. In television sound applications a convenient value is half the horizontal frequency, about 7.8 kHz.
For convenience in instrumentation and better tracking between encoder and decoder, the signal entering bandwidth limitation 54 in the encoder lO may be derived from the information bit-stream 82 instead of the output of spectral analysis 52. Such configuration is illustrated in Fig 4, with adaptive pre-emphasis 56, limitation circuit 54 and A-D
converter 72 rearranged as shown. A local D-A
converter lO0 converts the digital step-size information from bit-stream 82 into an analog emphasis control signal. The circuit arrangement of Fig 4 is particularly advantageous where the A-D converter 72 uses delta-sigma modulation so that the local D-A
converter lO0 is already contained within the A-D

~. . . .
:: :

~3~

converter and no extra local D-A converter will be necessary. Similarly the step-size information supplied to adaptive delta-modulator circuit 22 may be derived from the step-size information bit-stream 23.
This is again advantageous if A-D converter 26 uses delta-sigma modulation.
Instead of using a bandwidth limiting circuit 46 for limiting the bandwidth of step-size control signal applied to the adaptive delta-demodulator, D-A
converter 44 may contain the bandwidth limit~tlon.
Similarly, bandwidth limitation circuit 80 may be eliminated iP D-A converter 76 is similarly bandwidth limited.
Since it is desirable that the effect of a bit error should be a gain error of similar logarithmic magnitude for both large and small step-si~es, it is preferable to design the A D converter 26 and D-A
converter 44 so that the digital bit-stream 28 conveys the logarithm of the step-size. Similarly, the spectral information bit-stream preferably conveys the logarithm of the spectral information. In embodiments in which logarithmic and exponential circuits are inconvenient, it may be more practical to convey some other non-linear function of the step-size, such as the square root or the cube-root; such functions will not give perfectly uniform gain errors over the dynamic range of the system, but the extent of the variation will be much less than that resulting from a linear function.
For the same reasons discussed above for transmission of audio information, it is desirable to design an encoder-decoder system which conveys step-size information and spectral information at low bit .

~64~S;

rates of transmission and which can be implemented at low cost~ In choosing the scheme For ~-D and D-A
conversion for converters 26, ~4, 72 and 76 it is desirable to choose one that allows a low bit rate for the t~ransmission of step-size information. Preferably, such bit rate is small compared to the bit rate for transmission of audio data. The A-D or D-A conversion performed in converters 26, 72 and 44, 76 can be one of many schemes, including PCr~, delta modulation or delta-sigma modulation. ~hile a PCr~ system requires a lo~bit rate, expensive converters must be used so that it is undesirable to use PC~ in the converters. Delta-sigma modulation requires a somewhat higher bit rate (on the order of 5 to 10 kbit/sec) than PCM but it can oe implemented simply and at low cost. Furthermore, the bit rate required for delta-siqma modulation is still low compared with the bit rate for the transmission of audio data (on the order of 200 to 300 kbit/sec). Therefore, delta-sigma modulation is used in the preferred embodiment discussed below. A
description of delta-sigma modulation can be found in Delta Modulation Systems, Pentech Press Limited, London, 1975 by Raymond Steele Fig. 5 is a block diagram for a decoder - 25 system illustrating the preferred embodiment of the invention; the characteristics of most of the circuit blocks are defined for the system in Fig. 5. The system is particularly suitable for consumer use. The adaptive delta demodulator or audio decoder 42 comprises a pulse height modulator 202 and a leaky integrator 204. Pulse height modulator 202 multiplies the step-size control signal Vss by ~1 or -1 in accordance with the audio data bit stream, and supplies ( ~L~3~

the result to the leaky integrator 204. The leak time constant may be approximately 0.5 milliseconds which corresponds to a cut-off frequency of approximately 300 Hz. The integrator integrates the resulting signal to produce an analog audio signal. At frequencies below the frequency corresponding to the leak time constant, the system is strictly not delta but delta-sigma modulation.
In reference to Fig. 2~, the adaptive delta modulator 22 also includes a leaky integrator (not shown) with a cut-o~E frequency which is abou~ the same as the one in the decoder. The step-size derivation rneans 18 may be a slope detector which eesponds to the pre-processed input audio signal by deriving a control signal indicative of the slopes of the signal components of the audio input with frequencies above the cut-off frequency and the amplitudes of the signal components with frequencies below the cut-off frequency.
In the preferred embodiment, the step-size or slope data are transmitted by delta-sigma modulation and in the form of the logarithm of the required step-size or slope. The slope data are therefore decoded in the slope decoder 205 by passage through a low pass filter 2a6 (corresponding to D-A converter 4~ and bandwidth limitation 46 of Fig. 2B), which determines the bandwidth (and hence the rise-time) and ripple of the slope voltage. In the preferred embodiment, a 3-pole low pass filter is employed which causes the step--size control signal Vss to have a rise time of about 10milliseconds corresponding to a bandwidth of about 50 Hz. The slope voltage is then applied to an exponentiator 208 or anti-log. ciecuit, which may be, ~236~

for example, a bipolar ~ransistor. If the normalized mean level of the bit stream (or the duty-cycle measured over the rise-time of the low pass filter) is written as y, then Vss = Vo ex? ky where Vo and k are constants suitable for the partic-ular implementation I~ a practical value oE k is lO ln 2, this ds~inition gives an increase of 6 dB in step-size Eor every increase of 0.1 in y. Since y is confined to a range of 0 to l, the resultant maximum possible range of Vss is 60 dB.
The transmission of slope information in logarithm form red~lces the dynamic range conveyed in the slope data bit-stream from about 50 dB to about l9 dB, and spreads the effect of bit errors more uniformly across the dynamic range. Since Vss is confined by the low pass filter 206 to a bandwidth of about 50 Hz, bit errors lead to slow random amplitude modulation of the output audio. The audible disturbance produced by errors in the slope data bit-stream may be negligible.
It has been observed that uncorrected bit error rates of up to l in lO0 or so produce nearly imperceptible disturbance of music or speech~
The low pass filter thus converts the di~ital slope data into analog data and limits its bandwidth.
Low pass filter 206 therefore performs both the functions of both the D-A converter 44 and bandwidth limitation 46 of Fig. 2B. In reference to Fi~s. 2A, 2B
and 5, delay means 20 introduces such delay that the ; slope data are received by filter 206 before the 6~

corresponding audio data are received by the pulse height modulator 202. Sucn time difference compensates for the rise time of about 10 milliseconds of Vss. In such manner, the need for a delay circuit in the decoder is eliminated.
~ ig. 3~ illustrates a set of de-emphasis curves whic~ are comple,nentary to those of the pre-emphasis curves o~ ~ig. 3A. There are many ways to synthesi~e responses Oe this nature. The sliding band de-emphAsis 78 deFined in FicJ. 5 sho~ls one practical iinplementation oF tne c1e-emphasis characteristic. The system deEinitions for all the circuit blocks in Fig. 5 together with one set of values of the constants giving satisfactory res~lts are listed below:

leaky integrator 204 1 + sT

3-pole L.P. filter 206,214 1 + sT

20 exponentiator 208 V exp ky (slope decoder) exponentiator 216 f exp kx (spectru~ data) 1ST1 1+ST2 -1 25 sliding band de-emphasis 78 +
l+STl l+sT

fixed de-empnasis 118 ~6~

-2~-s is the complex freauency T = 0.5 milliseconds Tl is variable so that the variable frequency of the sliding band de-emphasis fl is given by:
1 1/(2 ~ Tl) = fO exp kx T2 = 5 microseconds T3 = SO microseconds T~ = 2 milliseconds T5 = 25 microseconds f = 4 kH2 V is scaling factor to suit the design oE the audio decoder.
x and y are the normalized mean levels of their respective bit-streams, i.e. the ~roportion of l's measured over the smoothing time of the 3-pole L.P. filter.
k = 10 ln 2 = 6.93 The spectrum decoder 212 comprising 3-pole filter 214 and exponentiator 216 is substantially the same as the slope decoder. It finds the normalized mean level x of the spectrum data input which conveys the logarithm of the variable frequency of the desired sliding band de-emphasis fl defined above, fl being different from the variable frequencies 86a-96a, 86a'-: 25 96a' of Figs. 3A, 3B. The spectrum decoder generates the exponent or anti-log. of the mean level and applies the resulting voltage or current to the sliding band de-emphasis 78. The emphasis control signal is even :

~L;23~

less affected by bit errors in transmission than the slope data control signal.
In delta modulation systems the sampling frequency is vastly greater than the minimum required by information theory. Non-audio spectral components in the output are at frequenc~ies well above the audio band and only an elementary low pass filter such as filter 118 is necessary.
Fig. 6 is a schematic circuit diagram showing a possible implementation of the system of Fig. 5. As shown in Fig. 6, the sliding band de emphasis circuit 78 employs a main path 78a with Eixed characteristlcs in parallel with a further path 78b with variable characteristics. The variable characteristics of the further path are controlled by the resistance of a variable resistance 252, which in turn is controlled by the emphasis control signal from spectrum decoder 2120 There is no systematic compression or expansion of the dynamic range; the further path is controlled ultimately by the spectrum of the input audio.
In reference to Fig. 2B, by limiting the bandwidths of the step-size and emphasis control signals, the characteristics of the delta demodulator 42 and de-emphasis 78 can only change slowly. Because they have slow~y changinq characteristics, the delta demodulator and de-emphasis are thereby rendered linear or quasi-linear. It makes little difference whether the demodulation is performed ahead of the de-emphasis or vice versa. This linear oe quasi-linear feature of the decoder system is even clearer in the case of the preferred embodiment in Fig. 5. Four processes are performed on the audio bit stream: pulse height modulation, leaky integration, slidinq band de-emphasis ~ r t ~3~5 and fixed de-empnasis. All four are linear or quasi-linear processes so that they can be performed in any order.
In Fig. 5, the pulse height modulator 202 can be a relatively sirnple circuit since it is required only to switch the sign of voltage Vss depending on the state of the audio bit-stream. Thus the modulator 202 may be made at low cost for consumer decoders.
However, multiplying the audio data by the step-size control siqnal at a diFferent point , say after slidinq band de-e~phasis but beEore Eixed de-emphasis, may have the advantage that quality oE the audio output is improved. This may be desirable for applications such as in broadcast stations and other professional equipment. iJhile the multiplication will have to be performed by a circuit more complex and therefore costlier than the type of pulse height modulator adequate for the arrangement of Fig. 5, the improvement in quality for professional applications may be well worth the additional cost. L~ultiplying the audio data at a different point is permissible because the four processes are eEfectively linear as explained above.
All such possible arrangements of the four processes are within the scope of this invention.
Instead of using a single 3-pole filter in the slope and spectrum decoders 205, 212, it is possible to use a two pole filter instead if an additional single pole filter is used to filter the output of exponentiators 20~, 216. Thus the filtering can be split into two steps: one before the exponentiation and the other one after. Any arrangement of filters may be used as long as the filter for filtering the slope or spectrum data before

3~

the exponentiation restricts the ripple in the filter output to a few percent of its mean value.
This invention together with tne invention of the companion application referenced above reduce the 5 transmission bit rate of the audio bit-stream to that comparable with or somewhat less than the bit rate required for a companded PCM system with comparable performance. The transmission bit rates for the encoder-decoder system of this invention may be in the 10 region oE 200 or 300 kbit per second. The transmission o~ spectral and step-size information may require about 10 or 2~ kbit per second and does not add significantly to the total transmission bit rate required for the encoder-decoder system of this invention. The encoder-15 decoder system of Figs. 2A and 2B, however/ retains theadvantages of delta modulation systems. The invention reduces and in many applications eliminates disturbing effects of bit errors. It and its components have high tolerance of errors. The receiving equipment (decoder) 20 is inexpensive. The system is efficient in usage of channel capacity so that more excess capacity will exist for ~lexibility to add additional channels or more bandwidth will be available to other signals such as video signals. The transmission equipment (encoder) 25 does not require special attention or require the use of non-complementary signal processing.
It will be apparent to those skilled in the art that the principles described herein are applicable not only to adaptive delta-modulation, but 30 to other adaptive ~-D and D-A coding systems, such as delta-sigma modulation, double integration delta-modulation, and PCM syste~s with variable reference voltages.

~3~

While tne invention has been described for the processing and transmission of audio signals, it will be understood that it may be used for the processing and transmission of other signals as well.
5 The ab~ve description of circuit implementation and method is merely illustrative~thereof and various changes in arrangements or other details of the method and implementation may be ~ithin the scope of the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A circuit for pre-processing a signal before the signal is subjected to a medium which introduces noise, said circuit altering the spectral composition of the signal before it is subjected to the medium to reduce the noise subsequently introduced, said circuit comprising:
means for analyzing the spectral composition of said signal, and for generating a control signal indicative of the regions in a frequency spectrum, if any, where the predominant signal components of the signal are concentrated, wherein said analyzing means includes a bandwidth limitation circuit for limiting the bandwidth of the control signal;
means responsive to said control signal for altering the spectral composition of said signal by amplifying or attenuating the spectral components of said signal by different amounts as a function of the control signal; and means for introducing delay to said signal before reaching said spectral composition altering means to compensate for the bandwidth limitation, wherein the noise subsequently introduced by the medium is reduced.

2. The circuit of claim 1, wherein said delay introduced also compensates for the time required for deriving the control signal.

3. A circuit for pre-processing a signal to prepare it for a medium whose noise level rises with the signal level, said circuit comprising:
means for analyzing the spectrum of the signal and for generating a control signal indicative of the regions in a frequency spectrum, if any, where the predominant components of the signal are concentrated; and means responsive to said control signal for applying emphasis to the spectral components of the signal, so that when the predominant signal components are concentrated within a first frequency range, at least some of the spectral components with frequencies above those of the predominant spectral components are boosted, and the predominant spectral components remain substantially unchanged and when the predominant signal components are concentrated within a second frequency range with frequencies above those of the first range, spectral components with frequencies above those of the predominant signal components are boosted but the predominant signal components are bucked, so that when the predominant signal components are concentrated in the second frequency range, the rise in noise level caused by the medium is reduced.

4. The circuit of claim 3, wherein the second frequency range has an upper end and a lower end, and wherein the emphasis applying means is such that:
(a) when the predominant spectral components are detected to be concentrated below a first predetermined frequency in the first frequency range, spectral components above the first predetermined frequency are boosted;
(b) when the predominant spectral components are detected to be concentrated below a second predetermined frequency but above the first predetermined frequency, spectral components with frequencies above those of the predominant spectral components are boosted, said second predetermined frequency being substantially at the lower end of the second frequency range; and (c) when the predominant spectral components are detected to be concentrated above the second predetermined frequency, spectral components with frequencies above those of the predominant spectral components are boosted, the predominant spectral components are bucked and spectral components with frequencies below those of the predominant spectral components remain substantially unchanged.

5. The circuit of claim 4, further comprising means for limiting the bandwidth of the control signal before application to the emphasis applying means.

6. The circuit of claim 5, further comprising means for introducing delay to said signal so that when the delayed signal reaches the emphasis applying means, the control signal, after being limited in bandwidth, is available for controlling the emphasis applied to the spectral components of the signal.

7. The circuit of claims 3 or 4, wherein said first predetermined frequency is approximately 500 Hz.

8. The circuit of claim 4, wherein the emphasis applying means is such that when the predominant spectral components are detected to be concentrated below the second predetermined frequency, substantially no bucking is applied to the spectral components with frequencies lower than the second predetermined frequency.

9. The circuit of claim 4, wherein the second predetermined frequency is approximately 2 kHz.

10. The circuit of claims 1 or 3, wherein the input signal is applied directly to the analyzing means for analysis of its spectrum and detection of its predominant spectral components.

11. The circuit of claim 1, wherein said noise introduced by the medium increases with the signal level.

12. The circuit of claim 11, wherein said medium includes an analog to digital conversion, and wherein said noise is quantizing noise introduced by the conversion.

13. A circuit for restoring the spectral composition of a signal, the spectral composition of said signal having been altered by the application of emphasis to its spectral components by different amounts as a function of an emphasis control signal derived from analysis of the spectral composition of said signal, wherein said emphasis control signal has a predetermined bandwidth, and wherein the signal has been delayed before its spectral composition is altered to compensate for the time required for the derivation of the bandwidth limited emphasis control signal, said circuit receiving spectral information of the signal corresponding to the emphasis control signal and the altered signal through a medium, said circuit comprising:
means responsive to said spectral information for generating a de-emphasis control signal; and means responsive to the de-emphasis control signal for restoring said spectral composition by applying de-emphasis to the spectral components of the altered signal, said de-emphasis being substantially complementary to the emphasis.

14. A circuit for restoring the spectral composition of signals by applying variable de-emphasis, said circuit receiving through a medium a signal whose spectral composition has been altered by the application of emphasis to its spectral components by different amounts as a function of the spectral composition of the signal and in response to an emphasis control signal before it is subjected to the medium, said emphasis control signal indicating regions in a frequency spectrum, if any, where the predominant components of the signal are concentrated, said medium having a noise level which rises with the signal level, said circuit also receiving through the medium spectral information of the signal, wherein said spectral information received is distinguishable from the corresponding signal, said circuit comprising:
means responsive to said spectral information for generating a de-emphasis control signal; and means responsive to said de-emphasis control signal for applying de-emphasis to the spectral components so that, when the predominant signal components are concentrated within a first frequency range, at least some of the spectral components with frequencies above those of the predominant spectral components are bucked, and the predominant spectral components remain substantially unchanged and when the predominant signal components are concentrated within a second frequency range with frequencies above those of the first range, spectral components with frequencies above those of the predominant signal components are bucked but the predominant signal components are boosted, so that when the predominant signal components are concentrated in the second frequency range, the rise in noise level caused by the medium is reduced.

15. The circuit of claim 14, wherein the second frequency range has an upper end and a lower end, and wherein the de-emphasis applying means is such that:
(a) when the predominant spectral components are detected to be concentrated below a first predetermined frequency in the first frequency range, spectral components above the first predetermined frequency are bucked;
(b) when the predominant spectral components are detected to be concentrated below a second predetermined frequency but above the first predetermined frequency, spectral components with frequencies above those of the predominant spectral components are bucked, the second predetermined frequency being substantially at the lower end of the second frequency range; and (c) when the predominant spectral components are detected to be concentrated above the second predetermined frequency, spectral components with frequencies above those of the predominant spectral components are bucked, the predominant spectral components are boosted and spectral components with frequencies below those of the predominant spectral components remain substantially unchanged.

16. The circuit of claim 15, wherein the de-emphasis control signal generated indicates the normalized mean level x of the spectral information and wherein the de-emphasis applied is defined by the relationship:

gain = where s is the complex frequency T1 = fo/(2.pi. exp kx) T2 is about 5 microseconds T3 is about 50 microseconds, and fo is about 4 kHz.

17. A circuit for restoring the spectral composition of a signal received from a medium whose noise level varies with the signal level, said circuit receiving through the medium the signal whose spectral composition has been altered by the application of emphasis to its spectral components by different amounts as a function of spectral information indica-tive of regions in a frequency spectrum, if any where the predominant signal components of the signal are concentrated, said circuit also receiving through the medium the spectral information, wherein the spectral information received is distinguishable from the signal, said circuit comprising:
means responsive to said spectral information for generating a de-emphasis control signal, said generating means including a bandwidth limitation circuit for limiting the bandwidth of the de-emphasis control signal; and means responsive to the bandwidth limited de-emphasis control signal for restoring said spectral composition by applying de-emphasis to the spectral components of the altered signal, and wherein the effects of errors introduced by the medium are reduced.

18. The circuit of claims 13, 14, 15, wherein said spectral information is received by the circuit in advance of the altered input signal by a predetermined and substantially fixed time interval.

19. The circuit of claim 17, wherein said spectral information is received by the circuit in advance of the altered input signal by a predetermined and substantially fixed time interval.

20. The circuit of claim 13, wherein said spectral information is received by the circuit in advance of the altered input signal by a predetermined and substantially fixed time interval and wherein the time interval has such magnitude that said signal and said bandwidth limited de-emphasis control signal have a timing relationship substantially the same as that between the unaltered signal and the emphasis control signal so that the de-emphasis applied is substantially complementary to the emphasis.

21. The circuit of claim 19, wherein the time interval has such magnitude that said signal and said bandwidth limited de-emphasis control signal have a timing relationship substantially the same as that between the unaltered signal and the emphasis control signal so that the de-emphasis applied is substantially complementary to the emphasis.

22. The circuit of claim 15, wherein said first predetermined frequency is approximately 500 Hz.

23. A circuit for restoring the spectral composition of a signal, said spectral composition having been altered as a function of an emphasis control signal, said emphasis control signal indicating regions in a frequency spectrum where the predominant components of the signal, if any, are concentrated, said circuit receiving the altered signal and spectral information of the signal through a medium wherein said spectral information is received by the circuit in advance of the altered signal by a predetermined and substantially fixed time interval, wherein said information is distinguishable from said altered signal, said circuit comprising:
means responsive to said spectral information for generating a de-emphasis control signal, said generating means including means for limiting the bandwidth of the de-emphasis control signal; and means responsive to the bandwidth limited de-emphasis control signal for restoring said spectral composition by applying de-emphasis to the spectral components of the altered signal;
wherein said substantially fixed time interval compensates for the rise time of the bandwidth limitation so that the bandwidth limited de-emphasis control signal is available to the de-emphasis applying means when the spectrally altered signal related to such de-emphasis control signal arrives at the de-emphasis applying means, and wherein the effects of errors introduced by the medium are reduced.

24. The circuit of claim 23, wherein the time interval has such magnitude that said signal and said bandwidth limited de-emphasis control signal have a timing relationship substantially the same as that between the unaltered signal and said emphasis control signal so that the de-emphasis applied is substantially complementary to the emphasis.

25. The circuit of claims 13, 14, 15, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of errors introduced by the medium.

26. The circuit of claim 17 or 23, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of errors introduced by the medium.

27. The circuit of claim 13, 14, or 15, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, and said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal.

28. The circuit of claims 13, 14, or 15, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, and said spectral information is transmitted to the circuit in the form of digital signals converted from analog signals by delta-sigma modulation and wherein said low pass filter also decodes the digital spectral information to provide an analog de-emphasis control signal.

29. The circuit of claims 13, 14, or 15, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, and said low pass filter has a transfer characteristic of the form (1+sT)-3, where s is the complex frequency and T is about 2 milliseconds.

30. The circuit of claims 13, 14, or 15, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, said low pass filter comprises three single pole filters each with a time constant of about 2 milliseconds, and wherein the spectral composition of the signal has been altered in accordance with an emphasis control signal indicative of the spectral content of the signal, wherein the spectral information is in the form of the logarithm of said emphasis control signal, said circuit further comprising an exponentiator for generating the de-emphasis control signal from the spectral information.

31. The circuit of claims 13, 14, or 15, wherein said means for generating the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, said low pass filter comprises three single pole filters each with a time constant of about 2 milliseconds, the spectral composition of the signal has been altered in accordance with an emphasis control signal indicative of the spectral content of the signal, wherein the spectral information is in the form of the logarithm of said emphasis control signal, said circuit further comprising an exponentiator for generating the de-emphasis control signal from the spectral information, and wherein said low pass filter comprises a single pole filter for filtering the output of said exponentiator and a two pole filter for filtering the spectral information and for applying the filtered spectral information to the exponentiator for generating the de-emphasis control signal.

32. The circuit of claims 13, 14 or 15, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, said low pass filter comprises three single pole filters each with a time constant of about 2 milliseconds, the spectral composition of the signal has been altered in accordance with an emphasis control signal indicative of the spectral content of the signal, wherein the spectral information is in the form of the logarithm of said emphasis control signal, said circuit further comprising an exponentiator for generating the de-emphasis control signal from the spectral information, and wherein said low pass filter comprises a filter for filtering the spectral information, said filter having such a response that it restricts the ripple in its output to a few percent of its mean level, said filter applying its output to the exponentiator.

33. The circuit of claims 17 or 23, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, and said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal.

34. The circuit of claims 17 or 23 wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, and said spectral information is transmitted to the circuit in the form of digital signals converted from analog signals by delta-sigma modulation and wherein said low pass filter also decodes the digital spectral information to provide an analog de-emphasis control signal.

35. The circuit of claims 17 or 23, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, and said low pass filter has a transfer characteristic of the form (1+sT)-3, where s is the complex frequency and T
is about 2 milliseconds.

36. The circuit of claims 17 or 23, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, said low pass filter comprises three single pole filters each with a time constant of about 2 milliseconds, and wherein the spectral composition of the signal has been altered in accordance with an emphasis control signal indicative of the spectral content of the signal, wherein the spectral information is in the form of the logarithm of said emphasis control signal, said circuit further comprising an exponentiator for generating the de-emphasis control signal from the spectral information.

37. The circuit of claims 17 or 23, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, said low pass filter comprises three single pole filters each with a time constant of about 2 milliseconds, the spectral composition of the signal has been altered in accordance with an emphasis control signal indicative of the spectral content of the signal, wherein the spectral information is in the form of the logarithm of said emphasis control signal, said circuit further comprising an exponentiator for generating the de-emphasis control signal from the spectral information, and wherein said low pass filter comprises a single pole filter for filtering the output of said exponentiator and a two pole filter for filtering the spectral information and for appllying the filtered spectral information to the exponentiator for generating the de-emphasis control signal.

38. The circuit of claims 17 or 23, wherein said means for generating the de-emphasis control signal also limits the bandwidth of the de-emphasis control signal to reduce the effects of transmission errors, said de-emphasis control signal generating means includes a low pass filter for limiting the bandwidth of the de-emphasis control signal, said low pass filter comprises three single pole filters each with a time constant of about 2 milliseconds, the spectral composition of the signal has been altered in accordance with an emphasis control signal indicative of the spectral content of the signal, wherein the spectral information is in the form of the logarithm of said emphasis control signal, said circuit further comprising an exponentiator for generating the de-emphasis control signal from the spectral information, and wherein said low pass filter comprises a filter for filtering the spectral information, said filter having such a response that it restricts the ripple in its output to a few percent of its mean level, said filter applying its output to the exponentiator.

39. The circuit of claims 13, 14 or 15, wherein said de-emphasis control signal is substantially equal to the emphasis control signal so that the de-emphasis applied is substantially complementary to the emphasis.

40. The circuit of claims 17 or 23, wherein said de-emphasis control signal is substantially equal to the emphasis control signal so that the de-emphasis applied is substantially complementary to the emphasis.

41. The circuit of claims 13, 14 or 15, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise.

42. The circuit of claims 16 or 17, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise.

43. The circuit of claim 20, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise.

44. The circuit of claims 13, 14, or 15, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise, and wherein said transmission medium includes an analog to digital conversion and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

45. The circuit of claims 16 or 17, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise, and wherein said transmission medium includes an analog to digital conversion and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

46. The circuit of claims 20, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise, and wherein said transmission medium includes an analog to digital conversion and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

47. The circuit of Claim 3, wherein said circuit alters the signal in preparation for transmission through a medium, wherein the medium introduces noise which increases with the signal level.

48. The circuit of Claim 47, wherein said transmission medium includes an analog to digital conversion, and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

49. The circuit of Claims 4, wherein said circuit alters the signal in preparation for transmission through a medium, wherein the medium introduces noise which increases with the signal level.

50. The circuit of Claim 49, wherein said transmission medium includes an analog to digital conversion, and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

51. The circuit of Claim 14 wherein said spectral information is received by the circuit in advance of the altered input signal by a predetermined and substantially fixed time interval and wherein the time interval has such magnitude that said signal and said bandwidth limited de-emphasis control signal have a timing relationship substantially the same as that between the unaltered signal and the emphasis control signal so that the de-emphasis applied is substantially complementary to the emphasis.

52. The circuit of Claim 15 wherein said spectral information is received by the circuit in advance of the altered input signal by a predetermined and substantially fixed time interval and wherein the time interval has such magnitude that said signal and said bandwidth limited de-emphasis control signal have a timing relationship substantially the same as that between the unaltered signal and the emphasis control signal so that the de-emphasis applied is substantially complementary to the emphasis.

53. The circuit of Claim 51, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise.

54. The circuit of Claim 52, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise.

55. The circuit of Claims 21 or 23, wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise.

56. The circuit of Claim 51 wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise, and wherein said transmission medium includes an analog to digital conversion and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

57. The circuit of Claim 52 wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise, and wherein said transmission medium includes an analog to digital conversion and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

58. The circuit of claims 21 or 23 wherein said transmission medium introduces noise which increases with the level of the signal and wherein the restoration of the spectral composition of the signal reduces such noise, and wherein said transmission medium includes an analog to digital conversion and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

59. The circuit of claims 14, 15 or 16, wherein said medium includes an analog to digital conversion and wherein said noise introduced by the medium is quantizing noise introduced by the conversion.

60. The circuit of claim 1, wherein the medium introduces noise whose level rises with the signal level, wherein said analyzing means detects the predominant spectral components, if any, of the signal and wherein said spectral composition altering means applies emphasis to the different spectral components of the signal so that spectral components with frequencies above those of the predominant spectral components are boosted, the predominant spectral components are bucked or remain substantially unchanged and spectral components with frequencies below those of the predominant spectral components remain substantially unchanged, so that the rise in noise level caused by the medium is reduced.

61. The circuit of claim 1, wherein the delay introduced is approximately in the range 5 to 20 milliseconds.

62. A system for pre-processing a signal to prepare it for a medium which introduces noise and for post-processing the signal after it has been subjected to the medium, said system comprising:
(a) a pre-processing circuit which alters the spectral composition of a signal comprising:
means for analyzing the spectral composition of said signal, and for generating an emphasis control signal indicative of the regions in a frequency spectrum, if any, where the predominant components of the signal are concentrated, wherein said analyzing means includes a circuit for limiting the bandwidth of the emphasis control signal;
means responsive to said emphasis control signal for altering the spectral composition of said signal by applying emphasis to the spectral components of said signal by different amounts as a function of the emphasis control signal to prepare it for the medium; and means for introducing delay to said signal before reaching said spectral composition altering means to compensate for the time required for limiting the bandwidth of the emphasis control signal; and (b) a post-processing circuit for restoring the spectral composition of the signal, said circuit receiving the signal after it has been subjected to the medium, said post-processing circuit also receiving through the medium the emphasis control signal or a signal derived from said emphasis control signal, said post-processing circuit comprising:
means responsive to said emphasis control signal or the signal derived therefrom for generating a de-emphasis control signal, said de-emphasis control signal generating means including a circuit for limiting the bandwidth of the de-emphasis control signal; and means responsive to the de-emphasis control signal for restoring said spectral composition by applying de-emphasis to the spectral components of the altered signal, so that the noise introduced by the medium is reduced.

63. The system of claim 62, wherein the delay introduced by the delay introducing means delays the signal such that the emphasis control signal or the signal derived therefrom reaches the post-processing circuit a predetermined time period before the pre-processed signal reaches the post-processing circuit, said time period selected to compensate for the time required for limiting the bandwidth of the de-emphasis control signal, so that the de-emphasis control signal is available to the de-emphasis applying means when the pre-processed signal reaches the de-emphasis applying means.

64. A system for pre-processing a signal to prepare it for a medium whose noise level rises with the signal level, and for post-processing the signal after it has been subjected to the medium, said system comprising:
(a) a pre-processing circuit which alters the spectral composition of the signal to prepare it for the medium, said pre-processing circuit comprising:

means for analyzing the spectrum of the signal and for generating an emphasis control signal indicative of regions in a frequency spectrum where the predominant components of the signal are concentrated; and means responsive to said emphasis control signal for applying emphasis to the spectral components of the signal, so that when the predominant signal components are concentrated in a first frequency range, at least some of the spectral components with frequencies above those of the predominant spectral components are boosted, the predominant spectral components remain substantially unchanged and when the predominant signal components are concentrated in a second frequency range with frequencies above those of the first range, spectral components with frequencies above those of the predominant spectral components are boosted but the predominant signal components are bucked; and (b) a circuit for post-processing the signal after it has been subjected to the medium to restore its spectral composition by applying variable de-emphasis, said circuit receiving through the medium the emphasis control signal or a signal derived therefrom, said post-processing circuit comprising:
means responsive to said emphasis control signal or the signal derived therefrom for generating a de-emphasis control signal; and means responsive to said de-emphasis control signal for applying de-emphasis to the spectral components so that when the predominant signal components are concentrated in the first range, at least some of the spectral components with frequencies above those of the predominant spectral components are bucked, and the predominant spectral components remain substantially un-changed and when the predominant signal components are concen-trated in the second range, spectral components with fre-quencies above those of the predominant spectral components are bucked but the predominant signal components are boosted, so that noise introduced by the medium is reduced.

65. The system of claims 62 or 64, wherein said de-emphasis applied by the post-medium circuit is substantially complementary to the emphasis applied by the pre-processing circuit so that the original spectral composition of the signal is substantially restored by the system.

66. The circuit of claim 1, wherein said medium includes an analog to digital encoding process by an encoder for encoding the spectrally altered signal into a first digital bit stream and a subsequent digital to analog decoding process by a decoder for decoding the bit stream into an analog signal, said circuit further comprising a second A-D
converter for converting said control signal into a second digital spectral control bit stream.

67. The circuit of claim 66, wherein the second A-D
converter includes a D-A converter which converts said second digital spectral control bit stream into an analog spectral control signal, and wherein said spectral composition altering means is responsive to said spectral control signal in analog form derived from the second digital spectral control bit stream.

68. A circuit for altering the spectral composition of a signal for use in a system, said system including a circuit for restoring the spectral composition of spectrally altered signals received through a medium, said restoring circuit also receiving through the medium spectral information related to the spectrally altered signals received, said restoring circuit including means for bandwidth limiting the spectral information and means for applying de-emphasis to the received spectrally altered signals in response to the bandwidth limited spectral information related to such signals to restore the spectral compositions of such signals, said altering circuit comprising:
means for analyzing the spectral composition of a signal, and for generating a control signal indicative of the spectral composition of said signal;
means responsive to said control signal for altering the spectral composition of said signal by applying emphasis to the spectral components of said signal by different amounts as a function of the control signal to provide a spectrally altered signal; and means for introducing delay to said signal before reaching said spectral composition altering means, said time delay being such that the control signal is available to the medium by a predetermined and substantially fixed time inter-val before the spectrally altered signal is available to the medium;
wherein said substantially fixed time interval compensates for the rise time of the bandwidth limiting means so that the bandwidth limited spectral information is available to the de-emphasis applying means when the spectrally altered signal related to such information arrives at the de-emphasis means in the restoring circuit, and wherein the effects of errors introduced by the medium are reduced.

69. The circuit of claim 15, wherein the de-emphasis applying means is such that when the predominant spectral components are detected to be concentrated below the first predetermined frequency, the spectral components below the first predetermined frequency remain substantially unchanged, and such that when the predominant spectral components are detected to be concentrated below the second predetermined fre-quency but above the first predetermined frequency, the pre-dominant spectral components and spectral components with frequencies below those of the predominant spectral components remain substantially unchanged.

70. The circuit of claim 15, wherein said second predetermined frequency is approximately 2 kHz.